The Collaborative International Dictionary of English v.0.48:

Oriel \O"ri*el\, n. [OF. oriol gallery, corridor, LL. oriolum
   portico, hall, prob. fr. L. aureolus gilded, applied to an
   apartment decorated with gilding. See Oriole.] [Formerly
   written also oriol, oryal, oryall.]
   [1913 Webster]
   1. A gallery for minstrels. [Obs.] --W. Hamper.
      [1913 Webster]

   2. A small apartment next a hall, where certain persons were
      accustomed to dine; a sort of recess. [Obs.] --Cowell.
      [1913 Webster]

   3. (Arch.) A bay window. See Bay window.
      [1913 Webster]

            The beams that thro' the oriel shine
            Make prisms in every carven glass.    --Tennyson.
      [1913 Webster]

   Note: There is no generally admitted difference between a bay
         window and an oriel. In the United States the latter
         name is often applied to bay windows which are small,
         and either polygonal or round; also, to such as are
         corbeled out from the wall instead of resting on the
         ground.
         [1913 Webster]

WordNet (r) 3.0 (2006):

oriel
    n 1: a projecting bay window corbeled or cantilevered out from a
         wall [syn: oriel, oriel window]

Moby Thesaurus II by Grady Ward, 1.0:

38 Moby Thesaurus words for "oriel":
   alcove, bay, bay window, bow window, carrel, casement,
   casement window, corner, cove, cranny, cubby, cubbyhole, cubicle,
   fan window, fanlight, grille, lancet window, lantern, lattice,
   light, louver window, niche, nook, pane, picture window, port,
   porthole, recess, roomlet, rose window, skylight, snuggery,
   transom, wicket, window, window bay, window glass, windowpane